The present invention generally relates to the treatment of contaminated water, and more particularly to the detoxification of cyanide-containing water.
An important factor to consider in any industrial process is its environmental impact. With the enactment of strict environmental legislation, a constant need exists for effective pollution control procedures. This is especially true regarding the decontamination/detoxification of water. Substantial research has been conducted involving methods for removing heavy metals, organic contaminants, particulates, and the like from water used in industrial/commercial processes.
With respect to the removal of contaminants, an unusually difficult problem involves the removal of free cyanide from water so that the water may be detoxified. The term "free cyanide" as used herein shall include but not be limited to HCN and/or free, dissolved CN.sup.- ions. For example, significant and substantial amounts of contaminated water containing free cyanide are generated in the gold mining/processing industry. Nearly all gold mines currently operating throughout the world treat substantial amounts of solid rock in order to recover small quantities of gold therefrom. It is currently estimated that a ton of gold ore typically contains about 0.04-0.20 ounces of gold. Thus, effective extraction processes are necessary in order to treat large amounts of solid rock in an effective and economical manner.
Two basic gold extraction procedures are used by most modern gold processors, namely, (1) the isolation of gold from powdered rock (ore) and (2) the isolation of gold from coarse-crushed rock (ore). With respect to process (1), mined rock is first crushed into a fine powder having an individual average particle size of about 200 U.S. standard mesh. The powder is then combined with a water solution containing free cyanide ions (CN.sup.-) therein (e.g. produced by combining water with NaCN, Ca(CN).sub.2, or other comparable materials known in the art for gold processing). As a result, the free cyanide ions in the solution react with the gold contained within the powdered rock in order to extract the gold therefrom. Extraction of the gold is accomplished by the production of a water-soluble gold cyanide complex from the free cyanide ions and the gold removed from the rock. This complex consists of NaAu(CN).sub.2 when NaCN is used and CaAu(CN).sub.3 when Ca(CN).sub.2 is used as indicated above. The solution with the gold cyanide complex therein is then treated in order to recover metallic gold therefrom. This is typically accomplished by the precipitation of metallic gold from the solution onto a bed of metallic zinc in accordance with the following reaction which, for example purposes, involves the precipitation of metallic gold from NaAu(CN).sub.2 : EQU NaAu(CN).sub.2 +2NaCN+Zn+H.sub.2 O.fwdarw.Na.sub.2 Zn(CN).sub.4 +Au+H.sup.+ +NaOH (1)
Gold is electronegative to zinc in cyanide solutions, thereby enabling zinc to act as a precipitating agent with respect to the water-soluble gold cyanide complex described above. Basic conceptual information on the above reaction and other information on gold processing is discussed in greater detail in Clennell, J. E., The Cyanide Handbook, McGraw-Hill, Inc., pp. 102-132 (1915) which is incorporated herein by reference.
Once the metallic gold is extracted and obtained, a slurry remains which consists of the treated, powdered rock in combination with water having a high residual free cyanide level. The powdered rock and water are ultimately retained within large collecting ponds as described in greater detail below. In time, the powdered rock settles, thereby separating the rock from the water. The water at this stage contains about 70-180 ppm of free cyanide therein, and is thereafter recycled back into the gold processing system.
However, significant environmental problems occur with respect to the ponds of cyanide-containing water which remain outside in the environment (at least temporarily). At the cyanide concentration levels indicated above (e.g. about 70-180 ppm), the pond water is lethal to animal life (e.g. birds and water fowl). In prior years, substantial amounts of water fowl have died after drinking from ponds having cyanide-containing water therein produced in accordance with the foregoing process. To control this problem in the United States, the U.S. Environmental Protection Agency has prepared voluntary guidelines suggesting that gold producers reduce free cyanide levels in treatment water to less than about 50 ppm. Various conventional methods exist for accomplishing this reduction. These methods will be described in greater detail below.
Regarding process (2), low grades of gold ore are crushed into individual portions of rock and arranged in piles, with each pile being placed on an impermeable pad (e.g. made of rubber). Each individual portion of rock is about 1-4 inches in diameter. In addition, each pile is typically about 30-50 ft. high and occupies about 10.sup.7 to 3.times.10.sup.7 ft.sup.3. A cyanide ion-containing solution of the type described above in process (1) is then allowed to trickle downwardly through the pile and into the individual portions of rock which are substantially porous. This enables penetration of the cyanide solution throughout the interior of the rock. Extraction of the gold is again accomplished by the production of a water-soluble gold cyanide complex from the free cyanide ions and the gold removed from the rock. Metallic gold is thereafter recovered from the gold cyanide complex as indicated above in process (1) and described generally in Clennell, J. E., supra. Once the metallic gold is extracted, the remaining solution which contains residual free cyanide therein is reused on other piles of rock.
The immediate environmental problems caused by the large ponds described above in process (1) do not exist when process (2) is used. Specifically, large ponds of cyanide-containing water do not exist in process (2). Instead, process (2) generates ponds which are sufficiently small (e.g. about 1,000 ft.sup.2) to be covered with protective wire mesh. This is in direct contrast with the ponds described above in process (1) which may actually cover many (e.g. about 200) acres. However, problems result in process (2) when the piles of rock being treated have all of the desired gold removed therefrom. At that point, the rock is ready for return to the environment. However, any cyanide-containing water retained therein, as well as any other residual water left over in the small ponds after termination of mining operations must be treated so that the water may be detoxified. Specifically, the residual water must be treated so that the free cyanide levels therein are below about 0.2 ppm. This is the level necessary to place the water in compliance with most local ground water standards in the United States.
In order to detoxify the water used in gold processing operations (and in other processes which generate or produce considerable amounts of free cyanide-containing water), a number of chemical treatment methods have been developed. These methods basically fall into two categories: (A) oxidation; and (B) complexing. With respect to oxidation, various chemicals exist which oxidize CN.sup.- to CNO. These materials include SO.sub.2, H.sub.2 O.sub.2, and selected hypochlorites. The oxidation of CN.sup.- to CNO is beneficial in that CNO is less toxic than CN.sup.-.
However, oxidants have a number of inherent disadvantages. For example, the foregoing oxidants are significantly expensive (e.g. about $5-$10 dollars (U.S.) per 1,000 gallons of liquid being treated). In a large-scale gold processing operation, the use of oxidants can therefore become prohibitively expensive. Also, the foregoing oxidants may attack the cyanide-treated rock materials and liberate various heavy metals contained therein (e.g. Cu, Cr, As and others). This produces additional pollution problems and may, in fact, cause violations of applicable ground water regulations involving excessive heavy metal concentration levels. Finally, oxidants reduce the pH of the water being treated to about 6.9 or less which may also cause a lack of compliance with applicable ground water regulations.
The other conventional treatment method involves complexing free cyanide materials to form a variety of chemical complexes in which the materials (e.g. CN.sup.- ions) become "unavailable" in accordance with the alleged stability of the resulting complexes. However, the complexing agents must be selected very carefully or significant problems will result. For example, the use of sodium, potassium, or calcium salts (e.g. sulfates, nitrates, chlorides, and the like) will produce NaCN, KCN, and/or Ca(CN).sub.2 as reaction products. These products are very soluble in water, and will ultimately dissociate to produce toxic free cyanide ions in solution. Regarding the use of metal salts (e.g. sulfates, nitrates, chlorides, and the like) involving Zn, Ni, Cu, Co and/or Cd, other problems can occur. While Cu and Ni salts may produce relatively stable cyanide complexes (e.g. Cu(CN).sub.2.sup.- and/or Ni(CN).sub.4.sup.-2), such materials, themselves, constitute undesirable heavy metal contaminants which produce a variety of potential pollution problems. Also, the use of Cd salts offers two disadvantages. First, Cd forms a relatively weak cyanide complex (Cd(CN).sub.3.sup.-) which readily dissociates to produce free cyanide ions. Also, it introduces cadmium ions into the treatment process which can present significant pollution/toxicity problems. The same problem exists when Co salts are used. While stable complexes may result (e.g. Co(CN).sub. 6.sup.-4), such complexes offer considerable toxicity problems.
Tests conducted on the use of zinc salts for treating cyanide-contaminated water have indicated that a cyanide reaction product is produced consisting of Zn(CN).sub.2. However, in the presence of excess free cyanide ions, this material reacts to form Zn(CN).sub.4.sup.-2. While this compound is of minimal toxicity compared with Co, Cd, and/or Ni complexes, it is relatively unstable, and ultimately dissociates at a significant rate, thereby again producing free cyanide ions. Thus, zinc salts (e.g. sulfates) are not desirable for use alone or in conjunction with other compounds by gold processors and the like for cyanide decontamination.
The most commonly used cyanide removal/complexing agent involves ferrous sulfate (FeSO.sub.4), with the heptahydrate form (FeSO.sub.4 .multidot.7H.sub.2 O) being preferred. Unless otherwise indicated, use of the term "ferrous sulfate" herein shall signify use of the heptahydrate form. Ferrous sulfate generally reacts with free cyanide ions as follows: EQU Fe.sup.+2 +6CN.sup.- .fwdarw.Fe(CN).sub.6.sup.-4 ( 2)
In turn, the Fe(CN).sub.6.sup.-4 will thereafter react with additional Fe.sup.+2 ions to produce an insoluble ferrocyanide having the following formula: Fe.sub.2 [Fe(CN).sub.6 ]. This material is significantly stable, and does not readily dissociate to produce free cyanide ions. In addition, the use of ferrous sulfate heptahydrate involves much lower material costs compared with the oxidants described above. While the foregoing oxidants typically cost about 5-10 dollars (U.S.) per 1,000 gallons of contaminated liquid, ferrous sulfate heptahydrate costs about 1 dollar (U.S) per 1,000 gallons of contaminated liquid. However, the use of ferrous sulfate heptahydrate also presents a disadvantage which must be carefully considered. Specifically, ferrous sulfate heptahydrate typically contains about 1-2% free sulfuric acid (H.sub.2 SO.sub.4). When the ferrous sulfate heptahydrate reacts to form the cyanide compositions listed above, the sulfuric acid (which is a strong oxidant) may then attack any rock materials it comes in contact with (e.g. when piles of rock (ore) are treated after the removal of gold therefrom). As a result, heavy metals (e.g. Cu, Ni, and the like) which may be contained within the rock are liberated. This can cause significant environmental problems as noted above. In addition, when ferrous sulfate heptahydrate reacts to form the above-described cyanide complexes, it also liberates Fe.sup.+2 ions into the treated water, thereby causing possible failure of the water to meet applicable ground water standards for dissolved iron content. For example, in process (1) described above, the use of ferrous sulfate heptahydrate will create a high dissolved iron content in the slurry ponds of about 40 ppm. To meet most local United States ground water regulations, the dissolved iron content of the water must thereafter be reduced to about 0.3 ppm.
Accordingly, a need exists for a cyanide-containing water detoxification method which:
(1) is economical to use; PA1 (2) is effectively usable in gold processing operations which involve cyanide treatment materials; PA1 (3) produces a stable, non-polluting, non-toxic reaction product which, if desired, is readily isolated and removed from the remaining solution; and PA1 (4) avoids the production of harmful by-products, including but not limited to acids and heavy metals.
The present invention enables the detoxification of cyanide-contaminated water in a highly effective manner while satisfying all of these goals as described in greater detail below.